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Like Subject Delta, Sigma's suit has the standard features of an Alpha Series Big Daddy, including Plasmid ports in his arms and EVE tanks on his back. Sigma also has an arsenal of weapons to use along with Plasmids, including the signature Drill that Delta also wields. Sigma can also use the Minerva's Den-exclusive Ion Laser in addition to his other weapons. Unlike Delta's bronze colored suit, Sigma's is mostly colored silver around the helmet and chest.

It is revealed that Sigma is Charles Milton Porter, the creator of The Thinker. Porter had come to Rapture after his wife Pearl was killed in World War II. Adapting to Andrew Ryan's ideas, Porter met Reed Wahl, a brillant scientist, and the two worked together to create the Thinker for certain operations in Rapture, like dispatching trains and phone calls.

Porter and Wahl began to disagree on using the Thinker after creating it. While Wahl used it to predict outcomes of ball games or stock prices and profit from it, Porter began to use it to replicate his dead wife's voice. After Wahl started splicing, which drove him insane, he framed Porter by creating a fake Audio Diary that suggested Porter was an ally of Frank Fontaine. Andrew Ryan had Porter arrested and converted into an Alpha Series, while Wahl became obsessed with using the Thinker to solve the 'predictive equation.'

Although Jack and Subject Delta start off with finding Electro Bolt as their first Plasmid, Subject Sigma's first Plasmid is Telekinesis, which is already equipped from the start of the game.

Sigma is the eighteenth letter in the Greek alphabet. This means that Sigma was the eighteenth test subject taken from Persephone for Alpha Series conversion. Sigma is often used in Science and Mathematics to indicate summation of a set or series. This may explain why he begins with the Telekinesis Plasmid, as he is later in the series.

Sigma's suit is colored differently, resembling Subject Omega's but with a slight green tinge added to the black and grey tones.

Unlike other Alpha Series, Sigma is not bonded to one specific Little Sister. This is because Sigma never made it to the bonding stage before becoming active.[3]

A photograph in the ending video of Minerva's den shows the parts of Sigma's suit on the ground. Oddly, the boots are actually those of a Rosie, and not Alpha Series.

When Subject Sigma lifts the platform off of himself at the beginning of Minerva's Den, the player can see that the rendered right arm is actually a mirrored left arm, as apparent by the reversed Sigma letter. However in the image above, they are both facing one direction.

Although Subject Sigma is able to use Vita-Chambers in Minerva's Den, it is never explained how. However, it can be assumed that the Thinker helped to reprogram the Vita-Chambers for Subject Delta and Sigma as hinted by Reed Wahl in one of his audio diaries.

Oddly, Sigma's helmet in the ending of Minerva's Den is still seen glowing although it is removed. This may have been overlooked by the developers and didn't have time for changing.

Much like Subject Delta, Subject Sigma can still eat and drink, despite having a helmet on.

Sigma was previously mentioned in the removed audio diary Alpha Series, as the latest produced Protector at the time.

↑Radio Message by Brigid Tenenbaum in The Thinker: "H... Herr Sigma! It is Dr. Tenenbaum! With the computer disabled, Minerva's Den is falling to pieces! No pressure regulation, no air supply! You must defeat Wahl und bring the Thinker back to full power."